Developments in fibre optic cable installation have seen a move from traditional cable installation using heavy, reinforced cables to the use of blown fibre techniques.
Traditional installation of optical fibres underground has often subjected them to high pulling forces, as they are pulled into ducts by pulling machines. Since optical glass cannot survive more than a few percent elongation, it has to be protected from these tensile forces by strength members which may be steel ropes, glass-reinforced rods, or Aramid yarns. These make traditionally installed optical cables heavy, bulky, stiff, expensive, and also increase stripping and handling time.
Strength members are not required in blown fibre cables which instead use just airflow to install fibres into pre-installed tubes. This technique exposes fibres to almost zero stress, and so no reinforcement of the optical fibre is needed. This in turn confers advantages for multi-branching access networks in that fibre lifetime is preserved in full, there is much less splicing, dead fibres can be eliminated, network planning is simpler, less manpower is required for installation and future upgrades are quicker and easier.
As each of the preinstalled tubes is installed below ground level it is important that it is able to protect the fibres within from the external environment in order to prevent damage to the fibre and subsequent degradation of signal transmission. Currently, these tubes have blown fibre closures at their open ends which perform the tasks of sealing the tubes to prevent the ingress of silt and/or water and allowing connections with other tubes. If, in the process of installation or maintenance of the optical network, an installer attempts to blow a fibre through a tube using high pressure air (e.g. at 10 bar), and the tube has not been configured correctly, the closure may become pressurised resulting in possible explosion or injury to an installer if he attempts to open the closure when pressurised.
Pressure relief valves exist that are designed to relieve this excess pressure in the closure. For instance, document GB1601130 discloses a valve having a plunger which is moveable towards and away from an annular seat surrounding an inlet port. The plunger has a nose constituted in part by a flat annular metal portion adapted to be brought into metal-to-metal contact with a flat annular seating surface on the valve seat. When the plunger nose is in contact with the valve seat, a raised ridge of the valve seat enters into a recess in the nose of the plunger where it is pressed into resiliently deforming contact with part of an elastomeric sealing ring which occupies the recess, thus closing the port. The plunger is arranged to open at a predetermined pressure in order to relieve any excess pressure from the closure, ensuring that it is always safe to open.
The Applicant has noticed that current pressure relief valves have a major fault in that the body of the valve, i.e. the part sitting outside of the closure, is not sealed and contains large holes to allow air to escape. Since the underground chambers in which these closures are normally installed are often filled with silt and water, the Applicant has noticed that when such a valve is installed and the device remains inactive for a significantly long period of time, it is a common shortcoming for the body housing the plunger mechanism to become blocked with silt and/or water thus blocking the free movement of the plunger and the escape of pressurised air. Moreover, the ingress of water and/or silt may also give rise to a deterioration of sealing performance or valve sticking which results in the valve failing to open at the predetermined set pressures.
The Applicant has perceived the need of improving the pressure relief valves which are generally used in the field of fibre optic installations in order to increase reliability and the safety thereof, even when the closures are at least partially dipped in water and/or silt, so that the drawbacks mentioned above can be avoided or at least remarkably reduced.